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If you know the angle and the distance to the considered object and the camera (calibration) pparameters you can apply some funny geometry :
http://answers.opencv.org/question/1149/focal-length-and-calibration/
But it's a complex problem covering as far as I know a lot of computer vision subjects. You can't really assume that, taking in consideration the fact that the real world is 3D + perspective, you have a simple linear transformation between 2D photo and the objects represented.
You can read http://research.microsoft.com/apps/pubs/default.aspx?id=67278 :)
